Generating and transmitting demodulation reference signals

ABSTRACT

The present disclosure provides a solution which solves the problem of demodulation reference signal (DMRS) ambiguity by introducing separate, i.e. different DMRSs. This is especially the case for systems employing dynamic allocation of control and data signals to different PRBs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/198,449, filed on Jun. 30, 2016, which is a continuation of U.S.patent application Ser. No. 14/330,095, filed on Jul. 14, 2014, now U.S.Pat. No. 9,397,812, which is a continuation of International ApplicationNo. PCT/EP2012/050506, filed on Jan. 13, 2012, All of theafore-mentioned patent applications are hereby incorporated by referencein their entireties.

TECHNICAL FIELD

The present disclosure relates to a method for generating andtransmitting demodulation reference signals, and to a correspondingmethod in a receiver node. Furthermore, the disclosure also relates to atransmit device, a receiver device, a computer program, and a computerprogram product thereof.

BACKGROUND

In a cellular communication system, the downlink (DL) denotes thetransmission of the synchronization signals and information from a basestation (e.g. eNB) to a mobile user (e.g. a UE). On the uplink (UL) thetransmission direction is the opposite.

The DL of LTE cellular communication system is based on OrthogonalFrequency Division Multiplex (OFDM) transmission, using both time andfrequency resource units for information transmission. The OFDM signalincludes a set of complex sinusoids, called subcarriers, whosefrequencies are consecutive integer multiples of the basic (the lowestnon-zero) subcarrier frequency, where each complex sinusoid is weightedby a modulation symbol conveying certain number of information bits. Inthe time domain an OFDM symbol period includes an active part and acyclic prefix part. The duration of active part is the inverse of thebasic subcarrier frequency. A cyclic prefix (CP) is a signal appended atthe beginning of each OFDM symbol, and it includes a last portion ofactive OFDM symbol waveform.

The smallest time-frequency resource unit for DL LTE informationtransmission is called resource element (RE), occupying a single complexsinusoid frequency in an OFDM symbol. For the purpose of schedulingtransmissions to different UEs, the REs are grouped into larger unitscalled physical resource blocks (PRB). A PRB occupies a half (called“slot”) of a subframe, i.e. N_(symb) ^(DL)=7 (with normal cyclic prefixlength) consecutive OFDM symbol intervals in time domain, and N_(sc)^(RB)=12 consecutive subcarrier frequencies in frequency domain(occupying in total 180 KHz).

The two PRBs in a subframe occupying the same subcarriers form a PRBpair. Each PRB is labeled by a unique PRB number, which is an indexdenoting the position of the subband that the PRB occupies within agiven bandwidth. The PRBs are numbered from 0 to N_(RB) ^(DL)−1 within agiven bandwidth. Thus, the maximum LTE bandwidth (20 MHz) contains themaximum number (110) of PRBs, which is in LTE standard denoted by N_(RB)^(max,DL)=110. The relation between the PRB number n_(PRB) in thefrequency domain and resource elements (k, l) in a slot is given by

$n_{PRB} = {\lfloor \frac{k}{N_{sc}^{RB}} \rfloor.}$

The physical downlink control channel (PDCCH) is defined as a signalcontaining information needed to receive and demodulate user-specificinformation transmitted from the eNB to a UE through another signal,called physical downlink shared channel (PDSCH). The PDCCH istransmitted in the control channel region occupying a few OFDM symbolsat the beginning of each UE-specific basic transmission time interval of1 ms, called downlink subframe, which is the minimum time resource thatcan be allocated to a single UE. The number of OFDM symbols in eachcontrol channel region ranges from 1 to 3 as indicated by physicalcontrol format indicator channel (PCFICH) in each DL subframe.

Downlink control information (DCI) conveyed by PDCCH includesinformation necessary to demodulate related PDSCH or physical uplinkshared channel (PUSCH), such as time-frequency resource allocation, usedmodulation and coding scheme (MCS), etc. Error detection on DCItransmissions is provided through the Cyclic Redundancy Check (CRC). TheCRC bits, calculated from the DCI information bits, are attached to theDCI.

To demodulate the PDCCH signal the UE needs the estimate of thepropagation channel. The channel estimate is obtained from referencesignals (RS) transmitted through specially allocated REs. The RSs arealso used to define so-called antenna ports (APs). An AP is the basebandinput into the corresponding separate antenna system. An antenna systemincludes an RF chain connected to one or multiple antenna elements thatshould together produce a desired electro-magnetic radiation pattern. Ifthere is more than one transmit antenna port, and more than one receiveantenna port, the transmission is usually classified as Multiple InputMultiple Output (MIMO) transmission. The corresponding propagation pathsbetween each transmit antenna port and each receive antenna port jointlydefine a MIMO propagation channel. On the LTE DL the different RSs aretransmitted on different antenna ports, and thus can serve at the UE toidentify separate propagation paths in MIMO propagation channel. In thisway each RS defines a unique AP.

Up to eight DMRS antenna ports {7, 8, 9, 10, 11, 12, 13, 14} are definedto support up to eight spatial layers PDSCH transmission in LTE Rel-10.PDSCH is directly mapped onto the antenna ports defined by DMRS asillustrated in FIG. 2, showing the case of rank 2 transmission via AP 7and AP 8. In FIG. 1 the mapping between APs and physical antennasdepends on the implementation, and thus is not specified in thestandard.

There are three types of DL reference signals in LTE:

-   -   a) common reference signals (CRS) are broadcasted by a base        station to all UEs;    -   b) UE-specific channel state information reference signals        (CSI-RS); and    -   c) UE-specific demodulation reference signals (DMRS).

The first two kinds of RSs are used in the UE for calculating thechannel quality indicator (CQI), the parameter which is feedback to thebase station to help in determining which UE should be scheduled for thesubsequent transmission. The third kind of RS, the DMRS, is used todemodulate the data transmitted on PDSCH in the same PRB as that DMRS.Note however that in some transmission modes of PDSCH the DMRSs are nottransmitted, so only the CRS is used for the demodulation of PDSCH inthese transmission modes. Besides, the CRS s are the only referencesignals used for the demodulation of the PDCCH signals.

All of the RSs are characterized by a unique combination of theparticular time-frequency pattern of their REs and the modulationsequence whose elements modulate these REs. There are two possibletime-frequency patterns of DMRSs within a PRB pair, as shown in FIGS. 2Aand 2B.

According to FIGS. 2A and 2B, a DMRS modulation sequence {a_(p)(k)},k=0, 1, . . . , 11, can be represented by a 3×4 DMRS modulation matrixA_(p), as:

$\begin{matrix}{A_{p} = {\begin{bmatrix}{a_{p}(0)} & {a_{p}(3)} & {a_{p}(6)} & {a_{p}(9)} \\{a_{p}(1)} & {a_{p}(4)} & {a_{p}(7)} & {a_{p}(10)} \\{a_{p}(2)} & {a_{p}(5)} & {a_{p}(8)} & {a_{p}(11)}\end{bmatrix}.}} & (1)\end{matrix}$

The DMRS modulation sequence {a_(p)(k)} in each of the scheduled PRBpairs is obtained by multiplying symbol-by-symbol its antenna port (AP)sequence {w_(p)(k)} with a PRB scrambling sequence {q(n_(PRB),k)}, i.e.

a _(p)(k)=w _(p)(k)q(n _(PRB) ,k),k=0,1, . . . ,11  (2).

The AP sequences are used to make the DMRSs that share a common timefrequency pattern orthogonal, either over a slot, or over a subframe. AnAP sequence {w_(p)(k)} can be defined through the concatenation ofcolumns of the matrix W_(p),

$\begin{matrix}{{W_{p} = \begin{bmatrix}{w_{p}^{\prime}(0)} & {w_{p}^{\prime}(1)} & {w_{p}^{\prime}(2)} & {w_{p}^{\prime}(3)} \\{w_{p}^{\prime}(3)} & {w_{p}^{\prime}(2)} & {w_{p}^{\prime}(1)} & {w_{p}^{\prime}(0)} \\{w_{p}^{\prime}(0)} & {w_{p}^{\prime}(1)} & {w_{p}^{\prime}(2)} & {w_{p}^{\prime}(3)}\end{bmatrix}},} & (3)\end{matrix}$

where (as in the matrix (1)) each row contains the modulation symbols ofREs at the same subcarrier frequency. The symbols of matrix (3) aregiven in Table 1 below.

TABLE 1 AP sequence symbols for DMRS ports 7 to 14 Antenna port pw_(p)′(0) w_(p)′(1) w_(p)′(2) w_(p)′(3) (a) when (n_(PRB) mod 2) = 0 7 11 1 1 8 1 −1 1 −1 9 1 1 1 1 10 1 −1 1 −1 11 1 1 −1 −1 12 −1 −1 1 1 13 1−1 −1 1 14 −1 1 1 −1 (b) when (n_(PRB) mod 2) = 1 7 1 1 1 1 8 −1 1 −1 19 1 1 1 1 10 −1 1 −1 1 11 −1 −1 1 1 12 1 1 −1 −1 13 1 −1 −1 1 14 −1 1 1−1

The structure of the AP sequences {w_(p)(k)} given by (3) implies that,at given subcarrier frequency, the propagation channel is consideredconstant over a subframe because only in that case the different APsoccupying the same time frequency positions can be orthogonallyseparated by correlation in the receiver. Such correlation implies thatthe DMRS REs are averaged to suppress the additive noise at thereceiver, resulting in a single channel coefficient at given subcarrierfrequency within a subframe.

Additionally, even the DMRS averaging over all subcarrier frequencieswithin a PRB pair is possible, because the LTE standard specifies thatthe proprietary precoding of antenna ports 7 to 14 at the base stationhas to be constant over all subcarrier frequencies of at least one PRBbandwidth. In this way the UE receiver can only see one propagationchannel coefficient within a PRB pair of an observed antenna port, evenif some antenna port precoding is done at the base station transmitter,as the precoding coefficient is included in all the estimatedpropagation channel coefficients.

The PRB scrambling sequence {q(n_(PRB),k)} is generated by taking a12-symbols long segment of a long complex (quadriphase) pseudo-randomsequence {r(m)}, m=0, 1, . . . , 12N_(RB) ^(max,DL)−1 that falls intothe observed PRB after mapping {r(m)} to all REs allocated to the DMRSsin the whole bandwidth.

Similarly as an AP sequence, a PRB scrambling sequence {g(n_(PRB),k)}can be defined through the concatenation of columns of the matrixQ(n_(PRB)),

$\begin{matrix}{{{Q( n_{PRB} )} = \begin{bmatrix}{r( {n_{PRB} + 2} )} & {r( {n_{PRB} + 2 + {3N_{RB}^{\max,{DL}}}} )} & {r( {n_{PRB} + 2 + {6N_{RB}^{\max,{DL}}}} )} & {r( {n_{PRB} + 2 + {9N_{RB}^{\max,{DL}}}} )} \\{r( {n_{PRB} + 1} )} & {r( {n_{PRB} + 1 + {3N_{RB}^{\max,{DL}}}} )} & {r( {n_{PRB} + 1 + {6N_{RB}^{\max,{DL}}}} )} & {r( {n_{PRB} + 1 + {9N_{RB}^{\max,{DL}}}} )} \\{r( n_{PRB} )} & {r( {n_{PRB} + {3N_{RB}^{\max,{DL}}}} )} & {r( {n_{PRB} + {6N_{RB}^{\max,{DL}}}} )} & {r( {n_{PRB} + {9N_{RB}^{\max,{DL}}}} )}\end{bmatrix}},} & (4)\end{matrix}$

where (as in the matrix (1)) each row contains the modulation symbols ofREs at the same subcarrier frequency. From (4) it follows that a PRBscrambling sequence {_(q)(n_(PRB),k)} can be defined as:

q(n _(PRB) ,k)=r(n _(PRB)+2−k mod 3+└k/3┘3N _(RB) ^(max,DL)),k=0,1, . .. ,11  (5),

where └x┘ denotes the largest integer not greater than x.

The PRB scrambling sequence depends on the cell ID and a UE-specificparameter which can have two possible values, which can be independentlyset by the base station at the beginning of each subframe. Thisparameter is sent to the UE via PDCCH. All UE-specific DMRS ports (7 to14) have a common PRB scrambling sequence, which can change fromsubframe to subframe, having one of totally two possible versions,depending of base station scheduler decisions.

It has been widely recognized that the control channel region for LTEPDCCH might be insufficient in future deployment scenarios where asignificant increase of the number of users in the system is expected.Additionally, in order to reduce transmission overhead in the futuresystems the CRS might be removed, making the demodulation of PDCCH notfeasible. The major direction in finding the way to increase thecapacity of PDCCH and reduce transmission overhead is to introduce aUE-specific control channel, so-called e-PDCCH, which is supposed to bedynamically scheduled by the base station to each individual UE, in asimilar way as it is routinely done for the transmission of UE-specificinformation content on PDSCH.

A similar approach has already been adopted in LTE Rel. 10 for thedefinition of relay node operation. A relay node (RN) is defined assupplementary reception and transmission facility, whose task is toextend the coverage of base stations, both in the downlink and uplink.On the downlink, a RN receives control information from the base stationvia so-called relay physical downlink control channel (R-PDCCH). TheR-PDCCH conveys information necessary to demodulate related PDSCH at RN,or PUSCH transmission from the RN. The time-frequency resources ofR-PDCCH are fundamentally different from those of PDCCH: the R-PDCCHPRBs are scheduled and multiplexed with the PDSCH PRBs, both in time andfrequency.

The R-PDCCH can be demodulated using either CRS or DMRS referencesignals mentioned before; the type of reference signals is configured atthe eNB and then signaled to the higher layers software in the RN viathe data transmitted over PDSCH. Since the channel between an eNB and afixed RN varies very slowly in time domain, the PRBs scheduled forR-PDCCH remain optimum within a very long time period and therefore maybe signaled to the higher layers software in the RN, meaning that theresource allocation information is transmitted via the data bits overPDSCH, to be interpreted and implemented by higher layers software inthe RN.

A similar approach can be adopted for the design of e-PDCCH, so that thee-PDCCH is transmitted through the UE-specific specially scheduled PRBs.However, since the channel between an eNB and a UE varies much faster,both in time and frequency, than that between an eNB and a fixed RN, thePRBs scheduled for e-PDCCH can only work within a short time period,meaning that they should be indicated in a relatively frequent manner tothe desired UE. As the e-PDCCH is supposed to ultimately replace thePDCCH, and as the frequent resource allocation signaling to the higherlayers of the UE would consume capacity of PDSCH, it will be assumedfurther on that the information about UE-specific scheduled e-PDCCHresource allocation is not signaled to the UE.

Thus the key problem in designing a stand-alone, independently schedulede-PDCCH is how to detect in the UE the time-frequency resourcesdynamically allocated to each newly scheduled e-PDCCH transmission,where the allocated e-PDCCH resources can be localized or distributed infrequency domain.

An immediate solution is to use blind decoding at the UE, where the UEtries to detect its e-PDCCH at all possible frequency positions of thePRB pairs within given time-frequency and antenna ports search space.The blind detection includes demodulation of assumed e-PDCCH REs, toobtain the control channel information bits appended by CRC (CyclicRedundancy Check) bits calculated at the transmitter, followed by thecomparison of these demodulated CRC bits with the “reconstructed” CRCbits calculated by the UE from the demodulated control channelinformation bits. If the demodulated and the reconstructed CRC bits arethe same, the e-PDCCH is considered to be found and successfullydecoded.

Such a solution has a large implementation complexity in terms ofrequired number of operations. For example, assuming up to 100 PRB pairswithin the system bandwidth and an e-PDCCH with size of either 1 or 2 or4 PRB pairs, if the e-PDCCH is transmitted via one antenna port, thenumber of the maximal possible detection on an antenna port is as hugeas:

${{\begin{pmatrix}100 \\1\end{pmatrix} + \begin{pmatrix}100 \\2\end{pmatrix} + \begin{pmatrix}100 \\4\end{pmatrix}} = 3926275},{{{where}\begin{pmatrix}n \\m\end{pmatrix}} = {\frac{n*( {n - 1} )*\ldots*( {n - m + 1} )}{m*( {m - 1} )*\ldots*1}.}}$

Furthermore, if 4 antenna ports are considered as candidate antennaports, the total number of the maximal possible detection over allcandidate antenna ports will be as high as 4 times the above number.

Thus, it is an open problem how to design the downlink transmissionmethod which would allow for an efficient detection in the UE of thetime-frequency resources dynamically allocated and used by the basestation for the transmission of UE-specific control information.

According to a prior art solution, the frequency location for thee-PDCCH is indicated by a new DCI format transmitted in the PDCCHregion. The UEs first performs blind detection in the PDCCH region tofind the new DCI format and then determines whether there is e-PDCCH inthe data region according to the status of the new DCI format detection.Hence, the detection of e-PDCCH really relies on detecting the new DCIformat in the PDCCH region. This design is hierarchical, and isillustrated in FIG. 3. Obviously, this solution relies on explicitsignaling of scheduled e-PDCCH time-frequency resources (via PDCCH), soit does not solve the problem under assumptions as mentioned.

According to another prior art solutions a semi-static configurationmethod was proposed to indicate e-PDCCH time-frequency resources by highlayer signaling. However, since the semi-static configuration via highlayer signaling usually have much longer transmission time delay, it isdifficult to adapt to the time-varying fast fading channel.

SUMMARY

Therefore an aspect of the present disclosure is to provide a solutionwhich mitigates or solves the drawbacks and problems of prior artsolutions.

According to a first aspect, a method of transmitting a demodulationreference signal (DMRS) in a wireless communication system is disclosed.Time-frequency resource elements (REs) are used in the wirelesscommunication system for transmission of information. The methodincludes:

generating at least one first receiver specific DMRS, the at least onefirst receiver specific DMRS being associated with a data channel

generating at least one second receiver specific DMRS, the at least onesecond receiver specific DMRS being associated with a control channel,the control channel being associated with the data channel, the at leastone second receiver specific DMRS being different from the at least onefirst receiver specific DMRS;

transmitting the at least one first receiver specific DMRS with the datachannel, wherein time-frequency REs of a first physical resource block(PRB) are utilized for transmission of the data channel and the at leastone first receiver specific DMRS, and time-frequency REs of the firstPRB being utilized for transmission of the at least one first receiverspecific DMRS locate in a first set of RE positions in a PRB; and

transmitting the at least one second receiver specific DMRS with thecontrol channel, wherein time-frequency REs of a second PRB are utilizedfor transmission of the control channel and the at least one secondreceiver specific DMRS, time-frequency REs of the second PRB beingutilized for transmission of the at least one second receiver specificDMRS locate in a second set of RE positions in a PRB, and the second setof RE positions in a PRB overlap with the first set of RE positions in aPRB.

According to a second aspect, a method of utilizing a demodulationreference signal (DMRS) in a wireless communication system is disclosed.Time-frequency resource elements (REs) are used in the wirelesscommunication system for transmission of information. The methodincludes:

receiving at least one first receiver specific DMRS associated with adata channel, wherein time-frequency REs of a first physical resourceblock (PRB) are utilized for transmission of the data channel and the atleast one first receiver specific DMRS, and time-frequency REs of thefirst PRB being utilized for transmission of the at least one firstreceiver specific DMRS locate in a first set of RE positions in a PRB;

receiving at least one second receiver specific DMRS associated with acontrol channel, wherein the control channel is associated with the datachannel, the at least one second receiver specific DMRS is differentfrom the at least one first receiver specific DMRS, time-frequency REsof a second PRB are utilized for transmission of the control channel andthe at least one second receiver specific DMRS, time-frequency REs ofthe second PRB being utilized for transmission of the at least onesecond receiver specific DMRS locate in a second set of RE positions ina PRB, and the second set of RE positions in a PRB overlap with thefirst set of RE positions in a PRB; and

demodulating the control channel by using the at least one secondreceiver specific DMRS.

Each of the above mentioned methods may also be executed in anapparatus. The apparatus includes a memory retaining instructions and aprocessor. The instructions relate to steps of a method. The processoris coupled to the memory and is configured to execute the instructionsretained in the memory.

The present disclosure provides a solution which solves the problem ofDMRS ambiguity by introducing separate, i.e. different DMRSs. This isespecially the case for systems employing dynamic allocation of controland data signals to different PRBs.

Further applications and advantages of the disclosure will be apparentfrom the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings are intended to clarify and explain differentembodiments of the present disclosure in which:

FIG. 1 illustrates mapping of different antenna ports;

FIGS. 2A and 2B shows two possible time-frequency patterns of DMRSswithin a PRB pair; and

FIG. 3 illustrates the indication of dynamic resource allocation ofe-PDCCH from the control information conveyed in PDCCH.

DESCRIPTION OF EMBODIMENTS

To achieve the aforementioned and other aspects, the present disclosurerelates to a method in a transmit node and to a corresponding method ina receiver node. The method in a transmit node according to thedisclosure comprises the steps of: generating at least one firstreceiver specific demodulation reference signal (PDSCH-DMRS) associatedwith a receiver specific data channel signal (PDSCH); generating atleast one second receiver specific demodulation reference signal(e-PDCCH-DMRS) associated with a receiver specific control channelsignal (e-PDCCH); transmitting said receiver specific control channelsignal (e-PDCCH) concurrently with the associated second receiverspecific demodulation reference signal (e-PDCCH-DMRS); and transmittingthe receiver specific data channel signal (PDSCH) concurrently with theassociated first receiver specific demodulation reference signal(PDSCH-DMRS). Concurrently in this context should be interpreted asmeaning “at the same time”.

The method in a receiver node according to the disclosure comprises thesteps of: receiving at least one receiver specific control channelsignal (e-PDCCH) generated according to the method above and anassociated second receiver specific demodulation reference signal(e-PDCCH-DMRS); receiving at least one receiver specific data channelsignal (PDSCH) generated according to the method above and an associatedfirst receiver specific demodulation reference signal (PDSCH-DMRS);demodulating the receiver specific control channel signal (e-PDCCH) byusing the associated second receiver specific demodulation referencesignal (e-PDCCH-DMRS); and using information derived from the step ofdemodulating the receiver specific control channel signal (e-PDCCH) fordemodulating the receiver specific data channel signal (PDSCH).

According to an embodiment of the disclosure the method in the transmitnode further comprises the step of: dynamically allocating the receiverspecific data channel signal (PDSCH) and the receiver specific controlchannel signal (e-PDCCH) to different PRBs.

According to yet another embodiment of the disclosure the method in thetransmit node includes: generating the first receiver specificdemodulation reference signal (PDSCH-DMRS) by using a first sequence ofsymbols {a_(p)(k)}, k=0, 1, . . . , L−1 for modulating time-frequencyresource elements at fixed positions within physical resource blocksused for transmission of data channel signals (PDSCHs) at a transmitantenna port p; and generating the second receiver specific demodulationreference signal (e-PDCCH-DMRS) by using a second sequence of symbols{b_(u,p)(k)}, k=0, 1, . . . , L−1 for modulating time-frequency resourceelements at fixed positions within physical resource blocks used fortransmission of e-PDCCHs at a transmit antenna port p. The secondsequence of symbols {b_(u,p)(k)} is a u-th sequence from a set of Usequences that can be allocated to one or more receiver nodes (such asUEs) in said wireless communication system.

As the DMRSs are transmitted in the same PRBs as the correspondinge-PDCCH or PDSCH signals, searching for a receiver-specific DMRSmodulation sequence at the receiver, in all PRBs within a giventime-frequency search space, and in all possible antenna ports that canbe used for the transmission of DMRSs, can result in identifying thePRBs allocated for the transmission of either e-PDCCH or PDSCH signal.The major problem in that case would be that the receiver could notdetermine whether the detected PRBs are allocated to the e-PDCCH or tothe PDSCH signal, as in the current LTE system any UE-specific DMRSmodulation sequence is the same for both of these signals.

In order to resolve the DMRS ambiguity the present disclosure introducesseparate, i.e. different DMRSs for e-PDCCH and for PDSCH, respectively,such that both are UE-specific, both are using the same time-frequencyREs, but are using different DMRS modulation sequences. Thee-PDCCH-specific DMRS modulation sequence would allow the UE tounambiguously identify detected the PRB allocated to the e-PDCCH.

To reduce the complexity of searching the e-PDCCH DMRS, as well as tomaintain compatibility with legacy UEs, i.e. previous versions of theLTE standard, it is beneficial that the existing LTE DMRS modulationsequences {a_(p)(k)}, described by equation (1), are allocated to thePDSCH, while the e-PDCCH-DMRS modulation sequence is a new modulationsequence {b_(u,p)(k)}. This means that a_(p)(k)=w_(p)(k)q(n_(PRB),k),k=0, 1, . . . , 11 where q(n_(PRB),k) is a PRB scrambling sequenceaccording to 3GPP LTE standard.

Preferably, the e-PDCCH-DMRS modulation sequence {b_(u,p)(k)}, k=0, 1, .. . , L−1 is obtained by multiplying symbol-by-symbol the PDSCH-DMRSmodulation sequence {a_(p)(k)}, k=0, 1, . . . , L−1 with a UE-specifice-PDDCH signature sequence {s_(u)(k)}, k=0, 1, . . . , L−1, i.e.

b _(u,p)(k)=a _(p)(k)s _(u)(k),k=0,1, . . . ,L−1  (6),

where index u labels a u-th sequence from a set of U signature sequencesthat can possibly be allocated to one or more receiver nodes in thesystem. The index u may be implicitly or explicitly signaled to one ormore receivers according to a further embodiment of the disclosure.

The number of symbols in a signature sequence L is 12, i.e. equal to thenumber of DMRS-REs in a PRB pair according to the latest version of theLTE standard (Rel. 10). It is, however, straightforward to use the aboveconstruction of e-PDCCH-DMRS modulation sequences with some other numberL of DMRS-REs in a PRB pair.

An e-PDCCH signature sequence can be also represented by a matrix S₁₁,e.g. as:

$\begin{matrix}{S_{u} = {\begin{bmatrix}{s_{u}(0)} & {s_{u}(3)} & {s_{u}(6)} & {s_{u}(9)} \\{s_{u}(1)} & {s_{u}(4)} & {s_{u}(7)} & {s_{u}(10)} \\{s_{u}(2)} & {s_{u}(5)} & {s_{u}(8)} & {s_{u}(11)}\end{bmatrix}.}} & (7)\end{matrix}$

In some deployment scenarios, the base station might decide to allocatethe different frequency positions to PRBs scheduled jointly within asubframe. Even in that case a single UE-specific e-PDCCH signaturesequence of L symbols can be associated to each such pair of PRBs, sothat the symbols are mapped to e-PDCCH DMRS REs as described by thee-PDCCH signature matrix (7).

According to an embodiment that allows for very simple generation ofe-PDCCH-DMRS modulation sequences, and for a simple searching procedurein the UE receiver, the e-PDCCH signature sequence can be a binarysequence with alphabet including two values, +1 and −1.

According to another embodiment the transmit node (e.g. base station orrelay node) transmits simultaneously multiple e-PDCCH and PDSCH signals,corresponding to multiple UEs, over a common set of antenna ports, i.e.the cases when U>1.

If all the UEs share the same scrambling sequence in the correspondingPDSCH-DMRS modulation sequences (what depends on the base stationscheduler decision), then using the same e-PDCCH signature sequence inall UE-specific e-PDCCH transmission makes it impossible for each of theinvolved UEs to tell whether the corresponding e-PDDCH signal is itsown, or it belongs to some other UE. This UE-ambiguity can be optimallyresolved by multiple, UE-specific orthogonal e-PDCCH signature sequencesas described in the following disclosure.

For example, the set of 11 orthogonal binary e-PDCCH signature sequencesof length 12 is given by the columns 2 to 12 of 12×12 Hadamard matrix H,where

$\begin{matrix}{H = {\begin{bmatrix}{+ 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} \\{+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1}\end{bmatrix}.}} & (8)\end{matrix}$

The first column of (8), i.e. the vector of all +1 s, can be interpretedas a transparent “PDSCH signature sequence”, which means that each ofthe e-PDCCH-DMRS modulation sequences generated using the signaturesequences (8) is orthogonal to the corresponding PDSCH-DMRS modulationsequence. The columns of H would remain orthogonal if the rows of H arepermuted.

However, in some deployment scenarios it might be beneficial that thee-PDCCH signature sequences are also orthogonal over a number of symbolssmaller than L. For example, the UEs that move at high speed, or havethe different frequency positions allocated to PRBs scheduled jointlywithin a subframe, might benefit from performing the e-PDCCH detectionindependently in each slot. In that case the interference from otherUE-specific e-PDCCH signature sequences would be minimized if alle-PDCCH signatures are orthogonal both over L symbols and over L/2symbols.

Such e-PDCCH signatures can be obtained, for example, from the L×Lorthogonal matrices that can be structured as:

$\begin{matrix}{{G = \begin{bmatrix}A & A \\A & {- A}\end{bmatrix}},} & (9)\end{matrix}$

where A is an L/2×L/2 orthogonal matrix. For example, we can define anorthogonal matrix A₀ as the 6×6 DFT matrix, i.e.

$\begin{matrix}{{A_{0} = \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & W_{6}^{1} & W_{6}^{2} & W_{6}^{3} & W_{6}^{4} & W_{6}^{5} \\{+ 1} & W_{6}^{2} & W_{6}^{4} & {+ 1} & W_{6}^{2} & W_{6}^{4} \\{+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} \\{+ 1} & W_{6}^{4} & W_{6}^{2} & {+ 1} & W_{6}^{4} & W_{6}^{2} \\{+ 1} & W_{6}^{5} & W_{6}^{4} & W_{6}^{3} & W_{6}^{2} & W_{6}^{1}\end{bmatrix}},} & (10)\end{matrix}$

where W₆ ¹=e^(i2π/6), i=√{square root over (−1)}. If we insert A₀ in (9)we obtain the 12×12 matrix G₀, given as

$\begin{matrix}{G_{0} = {\begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & W_{6}^{1} & W_{6}^{2} & W_{6}^{3} & W_{6}^{4} & W_{6}^{5} & {+ 1} & W_{6}^{1} & W_{6}^{2} & W_{6}^{3} & W_{6}^{4} & W_{6}^{5} \\{+ 1} & W_{6}^{2} & W_{6}^{4} & {+ 1} & W_{6}^{2} & W_{6}^{4} & {+ 1} & W_{6}^{2} & W_{6}^{4} & {+ 1} & W_{6}^{2} & W_{6}^{4} \\{+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} \\{+ 1} & W_{6}^{4} & W_{6}^{2} & {+ 1} & W_{6}^{4} & W_{6}^{2} & {+ 1} & W_{6}^{4} & W_{6}^{2} & {+ 1} & W_{6}^{4} & W_{6}^{2} \\{+ 1} & W_{6}^{5} & W_{6}^{4} & W_{6}^{3} & W_{6}^{2} & W_{6}^{1} & {+ 1} & W_{6}^{5} & W_{6}^{4} & W_{6}^{3} & W_{6}^{2} & W_{6}^{1} \\{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & W_{6}^{1} & W_{6}^{2} & W_{6}^{3} & W_{6}^{4} & W_{6}^{5} & {- 1} & {- W_{6}^{1}} & {- W_{6}^{2}} & {- W_{6}^{3}} & {- W_{6}^{4}} & {- W_{6}^{5}} \\{+ 1} & W_{6}^{2} & W_{6}^{4} & {+ 1} & W_{6}^{2} & W_{6}^{4} & {- 1} & {- W_{6}^{2}} & {- W_{6}^{4}} & {- 1} & {- W_{6}^{2}} & {- W_{6}^{4}} \\{+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {+ 1} & W_{6}^{3} & {- 1} & {- W_{6}^{3}} & {- 1} & {- W_{6}^{3}} & {- 1} & {- W_{6}^{3}} \\{+ 1} & W_{6}^{4} & W_{6}^{2} & {+ 1} & W_{6}^{4} & W_{6}^{2} & {- 1} & {- W_{6}^{4}} & {- W_{6}^{2}} & {- 1} & {- W_{6}^{4}} & {- W_{6}^{2}} \\{+ 1} & W_{6}^{5} & W_{6}^{4} & W_{6}^{3} & W_{6}^{2} & W_{6}^{1} & {- 1} & {- W_{6}^{5}} & {- W_{6}^{4}} & {- W_{6}^{3}} & {- W_{6}^{2}} & {- W_{6}^{1}}\end{bmatrix}.}} & (11)\end{matrix}$

The corresponding orthogonal e-PDCCH signature sequences of length 12,being also orthogonal over the intervals of 6 symbols, as well asorthogonal to the “PDSCH signature sequence” over the intervals of 6symbols, are given either by the columns 2 to 6, or by the columns 8 to12 of matrix G₀.

Another interesting special case of (9) can be obtained by using any ofquadriphase 6×6 orthogonal matrices A=A_(i), i=1, . . . , 4; for exampleas:

$\begin{matrix}{{A_{1} = \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {- i} & {+ 1} & {+ i} & {- 1} \\{+ 1} & {+ i} & {- 1} & {+ i} & {- i} & {- i} \\{+ 1} & {+ 1} & {- i} & {- 1} & {- 1} & {+ i} \\{+ 1} & {- i} & {+ i} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {+ i} & {- i} & {- 1} & {+ 1}\end{bmatrix}},} & (12) \\{{A_{2} = \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ i} & {+ 1} & {- i} & {- 1} \\{+ 1} & {- i} & {- 1} & {- i} & {+ i} & {+ i} \\{+ 1} & {+ 1} & {+ i} & {- 1} & {- 1} & {- i} \\{+ 1} & {+ i} & {- i} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {- i} & {+ i} & {- 1} & {+ 1}\end{bmatrix}},} & (13) \\{{A_{3} = \begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {- i} & {- i} & {+ i} & {+ i} \\{+ 1} & {+ i} & {- 1} & {+ 1} & {- i} & {- 1} \\{+ 1} & {+ i} & {+ 1} & {- 1} & {- 1} & {- i} \\{+ 1} & {- i} & {+ i} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- i} & {- 1} & {+ i} & {- 1} & {+ 1}\end{bmatrix}},} & (14) \\{A_{4} = {\begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ i} & {+ i} & {- i} & {- i} \\{+ 1} & {- i} & {- 1} & {+ 1} & {+ i} & {- 1} \\{+ 1} & {- i} & {+ 1} & {- 1} & {- 1} & {+ i} \\{+ 1} & {+ i} & {- i} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {+ i} & {- 1} & {- i} & {- 1} & {+ 1}\end{bmatrix}.}} & (15)\end{matrix}$

For example, if we insert A₁ in (9) the corresponding orthogonal e-PDCCHsignature sequences of length 12, being also orthogonal over theintervals of 6 symbols, as well as orthogonal to the “PDSCH signaturesequence” over the intervals of 6 symbols, are given by the columns 2 to6 and 8 to 12 of the 12×12 matrix G₁, given as:

$\begin{matrix}{G_{1} = {\begin{bmatrix}{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {- i} & {+ 1} & {+ i} & {- 1} & {+ 1} & {- 1} & {- i} & {+ 1} & {+ i} & {- 1} \\{+ 1} & {+ i} & {- 1} & {+ i} & {- i} & {- i} & {+ 1} & {+ i} & {- 1} & {+ i} & {- i} & {- i} \\{+ 1} & {+ 1} & {- i} & {- 1} & {- 1} & {+ i} & {+ 1} & {+ 1} & {- i} & {- 1} & {- 1} & {+ i} \\{+ 1} & {- i} & {+ i} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- i} & {+ i} & {- 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {+ i} & {- i} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ i} & {- i} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {- i} & {+ 1} & {+ i} & {- 1} & {- 1} & {+ 1} & {+ i} & {- 1} & {- i} & {+ 1} \\{+ 1} & {+ i} & {- 1} & {+ i} & {- i} & {- i} & {- 1} & {- i} & {+ 1} & {- i} & {+ i} & {+ i} \\{+ 1} & {+ 1} & {- i} & {- 1} & {- 1} & {+ i} & {- 1} & {- 1} & {+ i} & {+ 1} & {+ 1} & {- i} \\{+ 1} & {- i} & {+ i} & {- 1} & {+ 1} & {- 1} & {- 1} & {+ i} & {- i} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {- 1} & {+ i} & {- i} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- i} & {+ i} & {+ 1} & {- 1}\end{bmatrix}.}} & (16)\end{matrix}$

The orthogonal e-PDCCH signature sequences can be labeled by an indexu=0, 1, . . . , L−2 (from the set of U signature sequences), so aUE-specific e-PDCCH signature sequence is obtained by allocating acertain value of u to a specific UE. The allocation of e-PDCCH signaturesequences to specific UEs can be done, for example, by specifying in thestandard the index u as a function of some UE identification number inthe network, known as Radio Network Temporary Identifier (RNTI) in theLTE standard, which is signaled to each UE by the base station once theconnection is established. The other option is to send the index u tothe UE by higher layer signaling (through PDSCH) to be used in asubsequent scheduling interval. Therefore, both implicit and explicitsignaling of the index u or equivalent may be performed by the system.

Regarding the method in the receiver node, mentioned method according toan embodiment of the disclosure further comprises: searching for thesecond receiver specific demodulation reference signal (e-PDCCH-DMRS) inall PRBs within a given time-frequency search space so as to identifyPRBs allocated for receiver specific control channel signals (e-PDCCHs).Preferably, the search is performed in all antenna ports that are usedfor transmission of the second receiver specific demodulation referencesignal (e-PDCCH-DMRS) if multiple antenna ports are used in thetransmissions.

According to yet another embodiment the searching step further includes:

-   -   i. identifying a set of candidate e-PDCCH PRB pairs by searching        the given time-frequency search space for a UE-specific and        e-PDCCH-specific DMRS modulation sequence; and    -   ii. demodulating and performing CRC check for candidates        belonging to the set, and if a CRC check is positive for a        candidate—assume that the candidate is correct.

Moreover, the searching may further involve:

-   -   iii. performing blind detection on the remaining e-PDCCH PRB        pairs in the given time-frequency search space if step ii) above        does not result in a positive CRC check.

Hence, in other words: if there is more than one e-PDCCH signaturesequence, the e-PDCCH searching algorithm should be adapted to encompassthe corresponding additional detection statistics. Besides, thesearching algorithm can be done in several iterations. In the firstiteration the UE can identify the set of candidate, i.e. potentiale-PDCCH PRB pairs, by searching for its own e-PDCCH-DMRS modulationsequence over the whole given time-frequency search space; in the seconditeration the UE performs one-by-one demodulation of each candidatee-PDCCH PRB pair and performs the CRC check: if the CRC check ofselected candidate e-PDCCH PRB pair is positive, the e-PDCCH isconsidered successfully detected and decoded. If not, the UE move on toanother candidate e-PDCCH PRB pair in the set. If none of the candidatee-PDCCH PRB pairs produces a successful CRC check, the UE should performe-PDCCH blind detection on the remaining PRB pairs in the time-frequencysearch space.

If only one UE is scheduled at the time, the same e-PDCCH signature canbe shared between multiple UEs. In that case a basic implementation ofan e-PDCCH searching procedure in the receiver node may comprise of thefollowing steps:

-   -   1) Decomposing all received OFDM symbols within a subframe into        subcarriers with corresponding modulation symbols,    -   2) Selecting in the decomposed received signal the PRB pair in        the frequency domain which has not been searched;    -   3) Assuming certain DMRS port;    -   4) Re-modulating REs that are allocated to the assumed DMRS port        with the complex-conjugate of the corresponding PDSCH-DMRS        modulation sequence; such re-modulated RE-s represent the first        propagation channel estimate;    -   5) Re-modulating the first propagation channel estimate with the        complex-conjugate of the e-PDCCH signature sequence, to obtain        the second propagation channel estimate;    -   6) Summing all the samples of the first propagation channel        estimate, and then find the (squared) absolute value of the sum,        to obtain the first detection statistic;    -   7) Summing all the samples of the second propagation channel        estimate, and then find the (squared) absolute value of the sum,        to obtain the second detection statistic;    -   8) Finding the maximum detection statistic;    -   9) Comparing the maximum detection statistic with an estimated        noise-level threshold, to determine whether the observed PRB        pair contains a DMRS transmission;    -   10) Choosing, if the observed PRB contains a DMRS transmission,        the PRB affiliation (PDSCH or e-PDCCH) that corresponds to the        maximum detection statistic, otherwise repeat the procedure        starting from step 2;    -   11) Repeating, if the observed PRB pair was a PDSCH PRB pair,        the procedure starting from the step 2.

The above searching procedure can be easily adapted to the case when thedifferent UE-specific e-PDCCH signatures are assigned to differentfrequency positions of PRB pairs of a given UE. However, the searchingprocedure will be simpler if a single, common UE-specific e-PDCCHsignature is assigned to any frequency position of PRB pairs of a givenUE.

The above searching procedure is based on the aforementioned fact thatthe LTE standard specifies that the proprietary precoding of antennaports 7 to 14 at the base station has to be constant over all subcarrierfrequencies of at least one PRB bandwidth. Consequently, if the channelestimate samples within a PRB pair are summed together, a commonprecoding coefficient can be drawn out of sum, so it does not influencethe outcome of the comparison of detection statistics. From the samereason the e-PDCCH detection performances will be optimum if thepropagation channel is constant over a PRB pair.

Moreover, in the above searching procedure, the step of re-modulatingthe REs with the complex-conjugate of the corresponding PDSCH-DMRSmodulation sequence, includes the UE-specific scrambling sequence, asdescribe earlier. The UE-specific parameter of the scrambling sequencesin the existing LTE transmission modes is sent to the UE via PDCCH,which is demodulated by CRS. However, it has been assumed that there isno PDCCH and no CRS, which means that either the UE has to make searchfor e-PDCCH using both versions of the scrambling sequence, or theUE-specific scrambling sequences parameter has to be sent to the UE byhigher layer signaling (through PDSCH) to be used in a subsequentscheduling interval.

Furthermore, as understood by the person skilled in the art, any methodaccording to the present disclosure may also be implemented in acomputer program, having code, which when run in a computer causes thecomputer to execute the steps of the method. The computer program isincluded in a computer readable medium of a computer program product.The computer readable medium may include any memory, such as a ROM(Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM(Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM),or a hard disk drive.

The disclosure further relates to a transmit device and a receiverdevice corresponding to the above methods. It should be noted that thetransmit device and receiver device can be modified, mutatis mutandis,according to the different embodiments of aforementioned methods. Thedevices comprise the suitable means for providing the functionsdescribed above. These means may e.g. be: processing means, transmittingmeans, receiving means, memory means, buffer means, antenna means, etc.

Finally, it should be understood that the present disclosure is notlimited to the embodiments described above, but also relates to andincorporates all embodiments within the scope of the appendedindependent claims.

1. A method for communication in a wireless communication system,comprising: receiving at least one first receiver specific DMRSassociated with a data channel, wherein time-frequency REs of a firstphysical resource block (PRB) are utilized for transmission of the datachannel and the at least one first receiver specific DMRS, andtime-frequency REs of the first PRB being utilized for transmission ofthe at least one first receiver specific DMRS located in a first set ofRE positions in the first PRB; receiving at least one second receiverspecific DMRS associated with a control channel, wherein the controlchannel is associated with the data channel, the at least one secondreceiver specific DMRS is different from the at least one first receiverspecific DMRS, time-frequency REs of a second PRB are utilized fortransmission of the control channel and the at least one second receiverspecific DMRS, time-frequency REs of the second PRB being utilized fortransmission of the at least one second receiver specific DMRS locatedin a second set of RE positions in the second PRB, and the second set ofRE positions in the second PRB overlap with the first set of REpositions in the first PRB; and demodulating the control channel byusing the at least one second receiver specific DMRS.
 2. The methodaccording to claim 1, wherein a first sequence of symbols {a_(p)(k)},k=0, 1, . . . , L−1 is used for modulating time-frequency REs at a firstset of RE positions in a first PRB for generation of the at least onefirst receiver specific DMRS, the first PRB being used for transmissionof the data channel at a transmitting antenna port p, L is a positiveinteger.
 3. The method according to claim 2, wherein the first sequenceof symbols {a_(p)(k)}, k=0, 1, . . . , L−1 is defined as:a_(p)(k)=w_(p)(k)q(n_(PRB), k), k=0, 1, . . . , 11, where q(n_(PRB),k)is a PRB scrambling sequence.
 4. The method according to claim 1,wherein a second sequence of symbols {b_(u,p)(k)}, k=0, 1, . . . , L−1is used for modulating time-frequency resource elements at fixedpositions within a second physical resource blocks for generation of thesecond receiver specific demodulation reference signal, and the secondphysical resource being used for transmission of the control channel ata transmit antenna port p, L is a positive integer.
 5. The methodaccording to claim 4, wherein the second sequence of symbols{b_(u,p)(k)} being a u-th sequence selected from a set of U sequences, uand U being positive integers, the set of U sequences being capable ofbeing allocated to one or more receiver nodes in the wirelesscommunication system.
 6. The method according to claim 4, wherein thesecond sequence of symbols {b_(u,p)(k)}, k=0, 1, . . . , L−1 is obtainedby multiplying a first sequence of symbols {a_(p)(k)}, k=0, 1, . . . ,L−1 with a receiver specific signature sequence {s_(u)(k)}, k=0, 1, . .. , L−1.
 7. The method according to claim 1, wherein the at least onefirst receiver specific demodulation reference signal is modulated in away different from the way that the at least one second receiverspecific demodulation reference signal is modulated.
 8. The method ofclaim 1, wherein the control channel is an enhanced physical downlinkcontrol channel
 9. The method of claim 1, wherein the data channel is aphysical downlink shared channel.
 10. An apparatus, comprising: areceiver, configured to receive at least one first receiver specificDMRS associated with a data channel, wherein time-frequency REs of afirst physical resource block (PRB) are utilized for transmission of thedata channel and the at least one first receiver specific DMRS, andtime-frequency REs of the first PRB being utilized for transmission ofthe at least one first receiver specific DMRS located in a first set ofRE positions in the first PRB, and to receive at least one secondreceiver specific DMRS associated with a control channel, wherein thecontrol channel is associated with the data channel, the at least onesecond receiver specific DMRS is different from the at least one firstreceiver specific DMRS, time-frequency REs of a second PRB are utilizedfor transmission of the control channel and the at least one secondreceiver specific DMRS, time-frequency REs of the second PRB beingutilized for transmission of the at least one second receiver specificDMRS located in a second set of RE positions in the second PRB, and thesecond set of RE positions in the second PRB overlap with the first setof RE positions in the first PRB; and a processor, configured todemodulate the control channel by using the at least one second receiverspecific DMRS.
 11. The apparatus according to claim 10, wherein a firstsequence of symbols {a_(p)(k)}, k=0, 1, . . . , L−1 is used formodulating time-frequency REs at a first set of RE positions in a firstPRB for generation of the at least one first receiver specific DMRS, thefirst PRB being used for transmission of the data channel at atransmitting antenna port p, L is a positive integer.
 12. The apparatusaccording to claim 11, wherein the first sequence of symbols {a_(p)(k)},k=0, 1, . . . , L−1 is defined as: a_(p)(k)=w_(p)(k)q(n_(PRB),k), k=0,1, . . . , 11, where q(n_(PRB),k) is a PRB scrambling sequence.
 13. Theapparatus according to claim 10, wherein a second sequence of symbols{b_(u,p)(k)}, k=0, 1, . . . , L−1 is used for modulating time-frequencyresource elements at fixed positions within a second physical resourceblocks for generation of the second receiver specific demodulationreference signal, and the second physical resource being used fortransmission of the control channel at a transmit antenna port p, L is apositive integer.
 14. The apparatus according to claim 13, wherein thesecond sequence of symbols {b_(u,p)(k)} being a u-th sequence selectedfrom a set of U sequences, u and U being positive integers, the set of Usequences being capable of being allocated to one or more receiver nodesin the wireless communication system.
 15. The apparatus according toclaim 13, wherein the second sequence of symbols {b_(u,p)(k)}, k=0, 1, .. . , L−1 is obtained by multiplying a first sequence of symbols{a_(p)(k)}, k=0, 1, . . . , L−1 with a receiver specific signaturesequence {s_(u)(k)}, k=0, 1, . . . , L−1.
 16. The apparatus according toclaim 10, wherein the at least one first receiver specific demodulationreference signal is modulated in a way different from the way that theat least one second receiver specific demodulation reference signal ismodulated.
 17. A computer-readable medium comprising instructions that,when executed, cause an apparatus to implement the following steps:receiving at least one first receiver specific DMRS associated with adata channel, wherein time-frequency REs of a first physical resourceblock (PRB) are utilized for transmission of the data channel and the atleast one first receiver specific DMRS, and time-frequency REs of thefirst PRB being utilized for transmission of the at least one firstreceiver specific DMRS located in a first set of RE positions in thefirst PRB; receiving at least one second receiver specific DMRSassociated with a control channel, wherein the control channel isassociated with the data channel, the at least one second receiverspecific DMRS is different from the at least one first receiver specificDMRS, time-frequency REs of a second PRB are utilized for transmissionof the control channel and the at least one second receiver specificDMRS, time-frequency REs of the second PRB being utilized fortransmission of the at least one second receiver specific DMRS locatedin a second set of RE positions in the second PRB, and the second set ofRE positions in the second PRB overlap with the first set of REpositions in the first PRB; and demodulating the control channel byusing the at least one second receiver specific DMRS.
 18. The mediumaccording to claim 17, wherein a first sequence of symbols {a_(p)(k)},k=0, 1, . . . , L−1 is used for modulating time-frequency REs at a firstset of RE positions in a first PRB for generation of the at least onefirst receiver specific DMRS, the first PRB being used for transmissionof the data channel at a transmitting antenna port p, L is a positiveinteger.
 19. The medium according to claim 18, wherein the firstsequence of symbols {a_(p)(k)}, k=0, 1, . . . , L−1 is defined as:a_(p)(k)=w_(p)(k)q(n_(PRB),k), k=0, 1, . . . , 11, where q(n_(PRB),k) isa PRB scrambling sequence.
 20. The medium according to claim 17, whereina second sequence of symbols {b_(u,p)(k)}, k=0, 1, . . . , L−1 is usedfor modulating time-frequency resource elements at fixed positionswithin a second physical resource blocks for generation of the secondreceiver specific demodulation reference signal, and the second physicalresource being used for transmission of the control channel at atransmit antenna port p, L is a positive integer.